Fluorine Positional Isomerism Enables H‐Aggregation for High‐Performance Semicrystalline Polymer Donors in Organic Solar Cells

Constructing high-performance semicrystalline polymer donors is often hindered by the trade-off between crystalline order and donor-acceptor miscibility. Herein, we demonstrate that fluorine positional isomerism serves as a precise molecular handle to reconcile this conflict by modulating aggregation behaviors. Through regulating fluorine substitution from alkoxy ortho- (PFo) to meta- (PFm) and para- (PFp) positions, a distinct transition from J-aggregation to H-dominant aggregation is achieved. Contrary to the conventional preference for J-aggregates, the H-aggregated PFp achieves superior photovoltaic performance in this BDF-based polymer system, driven by the synergistic effects of HOMO stabilization, optimized packing, and matched interfacial energetics. Comprehensive dynamical studies, including vibronic analysis and transient absorption (TA) spectroscopy, reveal that H-aggregation maximizes π-π orbital overlap and optimizes phase separation, thereby suppressing non-radiative recombination and accelerating ultrafast charge transfer. Consequently, PFp-based binary devices deliver a champion efficiency of 18.54%. Furthermore, demonstrating the universality of this strategy, PFp acts as a potent morphology regulator in ternary blends, boosting the efficiency to an impressive 19.77%. This work establishes fluorine positional isomerism as a precise molecular handle to guide H-aggregation, synergistically optimizing energy levels and interfacial miscibility, offering an effective design strategy for high-performance semicrystalline polymer donors.